1. Field of the Invention
This invention relates to a semiconductive silicone rubber composition which cures into a silicone rubber having a stable electrical resistivity in the semiconductive region and thus is useful as semiconductive rolls such as charging rolls, transfer rolls and developing rolls in business machines such as copying and facsimile machines. It also relates to a semiconductive silicone rubber roll.
2. Prior Art
In the field of business machines, a variety of rubber materials including silicone rubbers, urethane elastomers, ethylene-propylene rubbers, and natural rubber are currently used to form rolls while composite materials of these rubbers are also available. The rubber materials are used in a variety of applications as electrically insulating materials, and with conductive agents added, they are also used as electroconductive materials for anti-static and electric conduction purposes. Nowadays, semiconductive materials having an intermediate resistivity range of 1.times.10.sup.3 to 1.times.10.sup.12 .OMEGA.-cm are increasingly used as roll materials for copying machines or the like. For semiconductive roll materials, the stability of resistivity is very important in most service environments where it is desirable to control current flow by the resistivity of the rubber material itself, for example, in order to impart an appropriate transfer ability. Also a high voltage of about 100 volts to 10 kilovolts is often applied across a semiconductive roll material in order to stabilize the current flow across the roll material or to provide a proper electric charge to an organic photoconductor (OPC) drum.
Most semiconductive roll materials use conductive carbon. For example, JP-A 306289/1994 uses furnace carbon black having a specific surface area of 25 to 50 m.sup.2 /g, a DBP oil absorption of 100 to 150 cc/100 g, and a toluene colored transmittance of 90 to 100%. JP-A 62241/1995 uses furnace carbon black having a .DELTA.DBP of 40 to 150 cc/100 g and a Dmod diameter of 250 to 400 nm Note .DELTA.DBP is a difference between a DBP oil absorption according to JIS K-6221 and a 24M.sub.4 DPB oil absorption according to ASTM D3493 which is a DBP oil absorption after compression, and the Dmod diameter is a diameter of carbon agglomerates.
However, use of carbon as the conductive agent has the problem that the stability of carbon is limited under the service environments of high voltages. For example, the carbon-to-carbon structure can be cleaved. Due to vaporization of carbon, the resistivity will deviate from the preset value during long-term use. These problems become outstanding as the voltage is increased and as the current is conducted more and more.
When conventional conductive carbon black is used, it is known that the current-to-voltage does not follow Ohm's law and becomes nonlinear due to the tunnel effect of the carbon structure (see L. K. H. van Beek and B.I.C.F. van Pul, J. Appl. Polymer Sci., 6, 651 (1962)). This nonlinear current-to-voltage is inadequate for OPC-related rolls including developing rolls and transfer rolls whose resistivity must be precisely controlled within the semiconductive region. As a solution to this problem, JP-A 192486/1994 discloses the addition of phosphates and JP-A 86205/1988 discloses the use of potassium titanate whiskers. These are not satisfactory in solving the above-mentioned current-to-voltage relationship.